Ferroelectric materials such as lead zirconate titanate (PZT) are utilized in forming ferroelectric capacitors. These capacitors are then used in the formation of ferroelectric RAM (FRAM) devices due to the capacitors' electrical properties of retention, read/write endurance, and speed of the write cycle.
As memory integrated circuits become more dense, low voltage operation becomes increasingly more important. Traditional methods of improving the polarization switching property of ferroelectric materials, such PZT, focus on changing the composition of the PZT by adding soft dopants such as Vanadium, Lead or other dopants. Lanthanum (La) doping has also been proposed as described in B. Yang, et al., “Low Voltage Performance of Pb(Zr,Ti)O3 Capacitors Through Donor Doping,” Applied Physics Letters, Vol. 71(24), Dec. 15, 1997. However, this approach has only pushed the driving voltage of the polarization switching down to between 1.5 to 2.0 volts. This, in turn, restricts the required voltage drive of FRAM devices to that range.
More recently, it was reported in N. Duan et al., “Enhancement of Dielectric and Ferroelectric Properties by Addition of Pt Particles to a Lead Zirconate Titanate Matrix,” Applied Physics Letters, Vol. 77(20), Nov. 13, 2000, that large amounts of platinum particles in a PZT matrix can result in improvements in the dielectric and ferroelectric properties of a PZT composite. A powder PZT/Pt composite was prepared by conventional solid state reaction by sintering of mixed powders, and dense compacts were formed from the composite for testing. The PZT/Pt composite material proposed by the article, however, is not a thin film. A thin film generally has a thickness that ranges from 1.0 nanometer to 1 micron. (K. N. Tu, et al., Electronic Thin Film Science: For Electronical Engineers and Materials Sciences, p. 1 (Macmillan Publishing Co. 1992). Rather, the PZT/Pt composite is a bulk material that contains micron-sized or larger particles of platinum in the PZT matrix. The bulk material is not suitable for forming ferroelectric capacitors in integrated circuits or FRAM devices. In ULSI technology, a ferroelectric film thicker than 1 μm (10,000 Å) in an integrated circuit has several disadvantages, including: (1) high cost; (2) difficult integration when a device is scaled down; and (3) difficulty in achieving low voltage applications.
Therefore, there remains a need for an improved ferroelectric capacitor, particularly a thin film ferroelectric capacitor, with improved polarization switching characteristics under low voltage drive and a method of manufacturing the same.